Fixed multiple issues; 3-prime ends of nucleotides now terminate correctly; strands not cleared unless atoms exist; strands regenerated with atomic model; cadnano models initialized with more correct azimuthal angle

Updated imports to be compatible with new package structure; updated paths to resources to use new get_resource_path() function

Added more complex nb types; increased intrahelical bond spring constant; tried and commented out increased angle spring constant; nothing really helped